1. Field of the Invention
The present invention relates to a frequency-multiplex optical transmission apparatus having a function of indicating a modulation signal level. More specifically, the present invention relates to a frequency-multiplex optical transmission apparatus having a circuit for indicating a modulation signal level of a laser diode for carrying out analog frequency-multiplex optical transmission.
2. Discussion of the Related Art
FIG. 3 is a diagram illustrating the characteristic of the optical output (P.sub.O) of a laser diode with respect to the drive current (I.sub.LD) thereof, and the relation between a waveform of the drive current and a waveform of the optical output.
In such a laser diode, the modulation degree m of the laser diode is expressed as follows: EQU m=I.sub.n /(I.sub.op -I.sub.th)
wherein I.sub.n represents an amplitude value, I.sub.op represents an operating point value, and I.sub.th represents a threshold value, in the drive current of the laser diode respectively.
Incidentally, in the case of a frequency-multiplexed signal, for instance, the modulation degrees of approximately 5% per channel for 40 transmission channels, and of approximately 10% per channel for 20 transmission channels have heretofore been known experimentally.
In this case, the voltage amount of the modulation signal applied to the laser diode has been measured to obtain the modulation degree.
FIG. 4 is a diagram showing an example of a conventional frequency-multiplex optical transmission apparatus. In this figure, an analog-frequency-multiplexed RF input signal from an input terminal 1 is applied to a laser diode 5 through an RF attenuator 2, an RF amplifier 3, and a splitter 4. The RF signal output from the amplifier 3 is splitted into an output terminal 6 by the splitter 4. When the laser diode 5 is subjected to optimum modulation, adjustment of the modulation signal level has been performed by measuring the RF signal at the output terminal 6 with a measuring instrument (not shown) such as a spectrum analyzer.
Due to measurement errors attributable to different measuring instruments, however, the adjusted modulation signal level tends to deviate from the optimum value thereof, resulting in deep modulation or shallow modulation to the laser diode 5.